Method of vacuum evaporation

ABSTRACT

A METHOD OF VAPOR DEPOSITING A UNIFORM LAYER OF MATERIAL ON THE SURFACE OF A PLANAR SUBSTRATE WHICH COMPRISES ROTATING SAID SUBSTRATE ABOUT ITS PERPENDICULAR AXIS WHILE MAINTAINING THE SUBSTRATE AT A PLANE INCLINE TO THE HORIZONTAL, WITH A SMALL SURFACE SOURCE OF EVAPORANT LOCATED BELOW SAID ROTATING SUBSTRATE, WITH THE RATIO OF THE RADIUS OF THE SUBSTRATE TO THE SOURCE-TO-SUBSTRATE DISTANCE BEING MAINTAINED FROM ABOUT 0 TO 1.0, SAID EVAPORANT SOURCE EMITTING A VAPOR FLUX OF SAID SUBSTRATE, WHILE MAINTAINING SAID SUBSTRATE AND EVAPORANT SOURCE UNDER VACUUM CONDITIONS DURING THE ENTIRE EVAPORATION CYCLE.

W. S. LITTLE, JR

METHOD OF VACUUM EVAPORATION July 17, 1973 4 Sheets-Sheet 1 Filed June 29, 1971 INVENTOR WILLIAM S. LITTLE JR.

ATTORNEY July 1-7, 1973 w. s. LITTLE, JR v v3, 6, 7

METHOD OF VACUUM EVAPORATION Filed June 29, 1971 4 Sheets-Sheet L02 BEST UNIFORMITY AT p=46.3

7' .002 fi'o 000 IOI July 17, 1973 Filed June 29,

p ll

w. s. LITTLE, JR 3,746,571

METHOD OF VACUUM EVAPORATION 4 Sheets-Sheet BEST UNIFORMITY AT FIG. 3

United States Patent 3,746,571 METHOD OF VACUUM EVAPORATION William S. Little, Jr., Rochester, N.Y., assignor to Xerox Corporation, Stamford, Conn. Filed June 29, 1971, Ser. No. 157,905 Int. Cl. C23c 13/00, 13/02, 13/04, 13/08 U.S. Cl. 117-105.4 4 Claims ABSTRACT OF THE DISCLOSURE A method of vapor depositing a uniform layer of material on the surface of a planar substrate which comprises rotating said substrate about its perpendicular axis while maintaining the substrate at a plane incline to the horizontal, with a small surface source of evaporant located below said rotating substrate, with the ratio of the radius of the substrate to the source-to-substrate distance being maintained from about 0 to 1.0, said evaporant source emitting a vapor flux of said substrate, while maintaining said substrate and evaporant source under vacuum conditions during the entire evaporation cycle.

BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for vacuum coating.

In many commercial applications of vacuum coating, the deposition of a film or coating must display a uniform thickness over relatively large surface areas. Although many approaches with respect to apparatus modifications may be considered in attempting to maintain uniformity of coating thickness, they all are limited to a certain extent by cost, practically, and the size or constraints of the vacuum chamber.

In conventional vacuum apparatus used for thermal evaporation, an evaporation boat containing the material to be evaporated is placed in a particular location within the vacuum chamber. The substrates to be coated are usually placed in a stationary location, usually above the evaporation source. The source material is then heated by any suitable means such as induction or resistance heating or electron bombardment. Particles of the evaporated material pass in straight lines from the source to the stationary substrate and deposit as a coating on the substrate positioned above the heated source.

The above method and apparatus, however, have some severe limitations and disadvantages when it is required to use a plurality of evaporation boats and/or to coat a plurality of substrates and/or to deposit an extremely uniform film on a sample that is nearly as large as the vacuum chamber. For example, when using more than one evaporation boat, all of the boats cannot be positioned in the same location to insure that a coating of a uniform thickness will be obtained. One method of avoiding this problem is to rotate a substrate or plurality of substrates about a centrally located evaporation source. This technique is described in U.S. Pat. 3,128,205. It can be seen, however, that the apparatus disclosed in U.S. Pat. 2,128,205 would probably require a relatively large vacuum chamber in view of the relatively complex arrangement which is required for rotating the substrates above the evaporation source.

In order to simplify conventional vacuum coating apparatus and to assure high thickness uniformity as well as simultaneous deposition on several samples up to 6 inches in diameter, K. H. Behrndt in the Transactions of 10th Natl. Vacuum Symposium, 1963; MacMillan Co. (New York), p. 379, in an article entitled Thickness Uniformity on Rotating Substrates has devised an improved method of vacuum coating. In brief, the substrates to be coated are positioned horizontally and are located in a circle concentric to a small area source. While the evaporated material is being deposited, the substrates to be coated are rotated about their axis. Behrndt illustrates mathematically which set of design parameters will yield the highest uniformity of coating thickness for varying shape and diameter of samples. Behrndt shows that uniform coatings may be obtained even under conditions where the substrate dimensions are nearly as large as the vacuum chamber dimensions if (1) The substrate is rotated at a constant velocity about its normal axis, and

(2) The substrate is located horizontally and off to one side so as not to be directly above the evaporation source.

He derives mathematically the optimum location for best uniformity, based on the various physical dimensions of the systems.

It has been discovered that an extremely high degree of film uniformity may be obtained by orienting the substrate in a non-horizontal position either directly above or eccentric to the evaporant source. It has been learned that not only a single optimum location for a substrate for the best coating uniformity exists, but rather, there exists a whole family of optimum plate orientations. Thus the operator has much more flexibility in choosing acceptable substrate positions for a given set of dimensions and constraints. In addition to improving uniformity of the coating thickness, it has been demonstrated that relatively large substrates can be uniformly coated in relatively small vacuum chambers, thereby increasing efficiency and reducing apparatus cost.

OBJECTS OF THE INVENTION It is therefore an object of this invention to provide an improved method of vacuum coating which overcomes the above noted disadvantages.

It is another object of this invention to provide a method of vacuum coating large planar substrates within a relatively small vacuum chamber.

It is another object of this invention to provide a method of vacuum deposition which exhibits a high deposition efficiency.

It is a further object of this invention to provide a method of vacuum deposition which exhibits a high unifarmity in coating thickness.

SUMMARY OF THE INVENTION This invention is directed to a method for thermal evaporation under vacuum conditions using a small surface source such as a crucible, boat, or the like. Through the use of the method and apparatus of the instant invention, means are provided for obtaining extremely uniform vacuum evaporated films or coatings on a substrate which is placed at a precisely determined position within a vacuum evaporation chamber above a small surface source of evaporant. The invention includes rotating the substrate about its normal axis with a constant angular velocity at a predetermined angle of tilt or inclination from the horizontal position. By using this technique, the substrate to be coated comes closer to the source in the region of smaller particle flux and farther from the source in the region of larger particle flux, so that the average material deposited during a large number of revolutions is nearly constant over a large surface area. One example of the instant invention allows the coating of 9 inch by 9 inch flat substrates in an :18 x 30 inch vacuum bell jar and maintaining a thickness variation within about 0.2 percent.

It is well known that the vapor flux radiating from an ideal surface source is inversely proportional to the square of the distance from the source. Further, in many practical cases it is found that the vapor flux is directly proportional to the cosine of the angle between the normal to the source surface and the direction of the vapor flow. This ideal type of source is used frequently in vacuum deposition analysis because surface sources can be closely approximated experimentally. They are also easily handled mathemtically if the source dimensions are small compared to the distance to the substrate. The evaporant is placed in a boat whose aperture is large compared to the height of the walls above the level of liquid evaporant. In the article by Behrndt referred to above, an expression is derived for the vacuum deposited film thickness on a rotating spherical or planar substrate. This article treats only those cases in which the substrate axis and the perpendicular to the source surface are parallel to each other (both vertical). Therefore, for planar substrates those results apply only for those plates lying in a horizontal plane somewhere above the source.

In the instant invention Behrndts analysis is generalized to include all possible orientations of the axis of rotation, but treats only planar substrates. While Behrndts results of planar substrates indicate a single optimum location of the substrate for best coating uniformity, the more general problem gives rise to a whole family of optimum plate orientations. Within each family there is a trade oif between the angle of inclination of the substrate and the angular displacement of the plates center from the vertical axis through the source. (It turns out, in one case for example, that the plate axis of rotation should be inclined about 46 with respect to the vertical when the plate is centered directly above the evaporation source. This optimum inclination angle changes to about 11 when the center of the plate is about 28 off to the side of the normal source and changes further to about when the plates center is 36 off to the side of the normal to the source. Each geometry gives rise to highly uniform deposition on a 12" diameter plate 24" away from the source. The operator may select any one of these plate locations, or any other intermediate plate location which is consistent with his vacuum chamber dimensions.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and various features of the above invention will become apparent upon consideration of the following detailed description of the invention especially when taken in conjunction with the following drawings wherein:

FIG. 1 is a schemattic view of one embodiment of the instant invention which illustrates by rectilinear coordinates the relative position of a planar substrate and an evaporation source.

FIG. 2 represents a family of surface profiles representing the variation in film thickness as a function of points on the substrate at various distances from the center. The entire family applies to a single location of the plates center. Each curve represents a diiferent angle of inclination of the plate.

FIG. 3 represents a family of source profiles of varying film thickness for a second position of a substrate and evaporation source.

FIG. 4 is a schematic sectional view of one embodiment of an apparatus suitable for rotating and varying V the angle of inclination of a substrate from the horizontal.

FIG. 5 is a schematic view of a second embodiment of apparatus suitable for rotating and inclining a substrate.

DETAILED DESCRIPTION OF THE INVENTION i -@Yldfili a Equation (1) The parameters X, Y and Z are the rectilinear coordinates of P as measured from the evaporant source in the coordinate system illustrated in FIG. 1, and

The calculations are repeated for various angles of inclination 8 of the substrate holder from the horizontal position. In this way, a complete set of families of film thickness profiles is plotted for a geometrical arrangement of fixed X, Y and Z of the coating apparatus. One may then select what appears to be the best profile for a particular application and then adjust the inclination angle 3 of the substrate to the corresponding value prior to evaporation.

For example, if the vacuum equipment makes it necessary to place the substrate center P directly above the substrate source, then Equation 1 is reduced to Equation 2.

tl dfl df F Equation (2) FIG. 2 illustrates the resulting family of surface profiles described by Equation 2. In the example, a substrate one foot in diameter is centered 2 feet directly above the source giving rise to a range of interest of S/R of from 0 to 0.25. The curves show that for this range of S/R, highly uniform films will result when 5 is about 46. The thickness variation under these conditions will be less than 0.2 percent over the entire substrate.

In the second case, if X=0.7 feet, Y=0.7 feet, and Z=l.73 feet, (making R=2 feet), then Equation 1 yields the profile shown in attached FIG. 3. In this case, the one foot diameter substrate inclined at a B of l1 would be uniformly coated to within 0.2 percent.

The table below shows those angles 3 which exhibit optimum uniformity for various values of the relative coordinates X/R and Y/R. The data in this table was obtained by examining multiple plots of Equation 1 for each (X/R, Y/R) pair and selecting the most uniform coating or film profile. The relation is used in conjunction with Equation 1.

is plotted as a function of S/R for various 8. The data in the table apply to the case where the substrate radius is 0.25 of the source-to-substrate distance R, but they are nearly the same as those data which would be obtained for plates whose radii lie anywhere in the range of 0.2. to 04 relative to the distance R.

It is apparent that the method described herein for determining optimum substate orientations is applicable to a wider range of substrate sizes than is covered by the particular data in the table.

The method can be successfully applied to planar substrates of radii ranging from 0.0 to 1.0 times R and higher.

The range of 0.1 to :8 is considered a preferred range for the relative substrate radius because, below 011, the substrate is far enough removed from the source of evaporant that a highly uniform coating can be obtained by simply placing the substrate in a horizontal position directly above the source. Above 0.8, the substrate is likely to extend so far to the side or so far downward toward the plane of the source that it would extend into regions where the fiux strength deviates from a true cosine law.

Apparatus suitable for rotating a planar substrate about its horizontal axis end and inclining or tilting the substrate to the appropriate angle of inclination from the horizontal axis is set forth in FIG. 4. This apparatus comprises two upright support members 10 and 11, containing a rotary power means 12a. The rotary power means 12a contains an outer housing 12 which contains therein the shaft member 13 containing bevel gear 14. Bevel gear 15 which is meshed with bevel gear 14 is connected to shaft 16 which is connected to spur gear 17 which is meshed with spur gear 18. Gear 18 is connected to meshed gear pair 19 and 20 and gear 20 connected to drive shaft 21 which is further connected to a drive motor which is not shown. Shaft member 13 is further connected to a substrate holder 22 by a flange 23. Substrate holder 22 contains a substrate 24 held in a fixed position by clip means 25 and 26. The entire rotating apparatus and substrate holder are adapted to be inclined at any appropriate angle from the horizontal plane by rotating this apparatus about an axis formed by pins 27 and 29 held in rotary position by bushings 28 and 30. The angle of inclination formed by moving power means 12a and substrate holder 22 is more clearly shown in FIG. 1 of the drawings.

In operation, the apparatus in FIG. functions to rotate substrate 24 about its horizontal axis by the rotation of drive shaft 21 actuating gear arrangement 19 and 20. This actuates gear arrangement 17 and 18 and causes shaft 16 to rotate gear arrangement 14 and 15, which result in shaft 13 rotating substrate holder 22 about the horizontal axis of substrate of 22.

An alternative means for rotating and inclining the substrate is illustrated by FIG. 6 in which a series of pulleys 40, 41, and 42 are driven by a chain member 43.

The instant invention is directed to thermal vacuum evaporation in which a small surface source such as a boat or crucible is heated by any suitable source to emit a deposition flux which is coated onto a substrate within a vacuum chamber. The present invention has utility for coating any suitable material onto any suitable substrate: For example, metallic, oxide, inorganic or organic materials, may be coated in thin films onto any suitable substrate materials such as glass, plastic, ceramic, metal, paper, etc. The vacuum conditions and boat temperatures naturally vary according to the vapor pressure of the deposition material, and type of substrate but generally lies in the range of about 10 to 10 torr are generally satisfactory.

A preferred application of the instant invention includes the vacuum deposition of a thin film of vitreous selenium and vitreous selenium alloys on a conductive substrate for use as a xerographic plate and drum.

Although specific components and proportions have been stated and the above description of the preferred embodiments of this invention, other suitable materials and procedures such as those listed above, may be used with similar results. In addition, other materials and changes may be utilized which synerigize, enhance or otherwise modify the method of the instant invention.

Other modifications and ramifications of the present invention would appear to those skilled in the art upon reading the disclosure. These are also intended to be within the scope of this invention.

What is claimed is:

1. A method of vapor depositing a uniform layer of material on the surface of a planar substrate which comprises rotating said substrate about its perpendicular axis while maintaining the substrate at a plane inclined to the horizontal, with a small surface source of evaporant located below said rotating substrate, with the ratio of the radius of the substrate to the source-to-substrate distance being maintained from about 0 to 1.0, said evaporant source emitting a vapor flux which deposits as a substantially uniform layer upon the surface of said substrate, while maintaining said substrate and evaporant source under vacuum conditions during the entire evaporation cycle.

2. The method of claim 1 in which the ratio of the radius of the substrate to the source-to-substrate distance is from about 0.1 to 0.8.

3. The method of claim 1 in which the inclined substrate is located substantially directly above the evaporant source.

4. The method of claim 3 in which the substrate is inclined at an angle of about 46 degrees.

References Cited UNITED STATES PATENTS 2,456,241 12/1948 Axler et al. 117107.1 3,128,205 4/1964 'Illsley 117107.1 3,628,994 12/1971 Blecherman 117107.1 3,297,475 1/1967 Flacche 117106 R 3,558,351 1/1971 Foster 117106 R 3,594,227 7/1971 Oswald 117106 A 3,632,406 1/ 1972 Clough et al. 117107.1

MURRAY KATZ, Primary Examiner M. SOFOCLEOUS, Assistant Examiner US. Cl. X.R. 

